1. Field of the Invention
This invention relates to flexible skirts that are used to form at least a part of the cushion sealing means on air cushion vehicles.
Air cushion vehicles may be fully skirted or may be of the sidewall type in which cushion containment is effected by rigid sidewalls and bow and stern flexible skirts. Whilst the present invention is applicable to both types of vehicle it will generally be discussed and described in connection with the fully skirted type of vehicle.
2. Description of the Prior Art
One flexible skirt that has been developed and extensively used on operational air cushion vehicles is of a two-part configuration, the upper part of the skirt being formed as an inflatable bag member that is stiffened by being inflated, while the lower part of the skirt is formed by a plurality of adjacent and independent flexible skirt elements known in the art as "fingers". Two such flexible skirts are disclosed in the specification of U.S. Pat. Nos. 3,502,168 and 3,618,695. The bag member is manufactured from flexible impermeable sheet material and has two opposed edges fixed to the vehicle hard structure along fixing lines that are spaced apart so that the sheet material extends between them to enclose the outlet end of a peripherally extending duct leading from a plenum chamber formed by the vehicle hard structure. In operation, pressurized air is delivered to the plenum chamber and flows through the peripherally extending duct to inflate the bag member. The section of the bag member that faces the cushion area beneath the vehicle is apertured to permit some of the air to escape in a controlled manner from the bag member into the cushion area to build up and sustain the cushion of pressurized air on which the vehicle is supported above the surface over which it operates.
The total cross-section area of the cushion-feed apertures in the bag member is a significant factor in establishing a desired pressure ratio between the air inflating the bag member and the air forming the cushion, the pressure ratio dictating the stiffness of the bag member. A bag member having a large total aperture area will give a low bag-to-cushion air pressure ratio and will be relatively soft, whilst a bag member having a small total aperture area will give a high bag-to-cushion air pressure ratio and will be relatively stiff. However, the pressure drop between the bag member and the cushion represents a power loss. In redesigning the SR.N4 craft so as to increase its size and payload capacity it was a requirement that the skirt be redesigned so as, amongst other things, to reduce the bag-to-cushion pressure ratio from 1.5 to 1.2 and hence reduce the power loss associated with feeding the cushion through the bag member by about 75%. This requirement demanded cushion feed apertures covering at least 12.5% and preferably 20% of the available area of that section of the bag member which faces inwardly towards the cushion.
The flexible sheet material from which the bag member is manufactured normally comprises elastomeric coated plain woven nylon fabric and the serious loss of strength which results from the cutting of holes in elastomeric coated fabrics is a major problem in the design of flexible skirts. Numerous tests on coupon specimens under axial tension have shown that even a relatively small hole will degrade the strength to something of the order of 30 or 40% of the basic strength of the material in the net section in way of the hole. Whilst the stress concentration factors implied by such results are very similar to those which can be calculated for conventional engineering materials, local yielding of ductile metal will normally result in a redistribution of load and the full effects of stress concentration only become apparent in fatigue. In contrast elastomeric coated fabrics, despite their high elongation under load, behave like brittle materials and catastrophic failure occurs as soon as the stress in the first continuous cord bounding a cushion-feed aperture reaches the ultimate tensile strength of that cord. This is because when the first continuous thread fails, its load has to be carried by the next adjacent continuous cord, which being overloaded also fails so that a self-propagating tear develops which will, in a short time, run the length of the bag member. It was apparent from the initial design stage that the requirement for increased cushion-feed aperture area could not be met using elastomeric coated plain woven nylon fabric whilst at the same time producing a bag member with the necessary structural integrity.